Oxygen is essential for innumerable natural and industrial processes. For many physical processes, the addition of excess oxygen, in place of air, typically enhances the kinetics—reaction rates and efficiencies—of these oxygen-dependent processes (Hendershot). In machining processes, in particular heavily loaded tribosystems (HLTS) such cutting and stamping processes, it is well established that oxygen; present in the atmosphere, dissolved in cutting/stamping fluid, and/or present as functional groups), is critical to the productivity or even efficacy of many cutting and stamping operations. In the absence of surface oxides, adhesion and complete welding of solid surfaces (tool-chip, tool-substrate) can occur (Buckley). This will negatively impact tool life, surface finish and machinability.
Conventional methods used to improve oxygenation in machining fluids include adding compounds containing oxygen functional groups including esters, alcohols, and aldehydes. For example, in U.S. Pat. No. 6,206,764, long chain alcohols, preferably derived from bio-based oils, are used to provide more effective lubrication and cooling action during the dicing or cutting of super hard ceramics. The enhancement over prior art is presumably (according to the present inventor) due to the presence of oxygenated (more specifically hydroxyl) species present in the oleic backbone, and demonstrates the importance of oxygen in dicing or grinding processes employing diamond blades.
Oxygenated compounds such as inorganic peroxides and ozone have been examined as lubricants in machining applications. Experiments with both hydrogen peroxide (H2O2) and sodium permanganate (NaMnO4) have demonstrated the benefits of oxygenating the cutting zone (Schey, Page 625). U.S. Pat. No. 3,670,606 (Blomgren) taught projecting an electrostatic discharge needle into a cutting zone, which produces ionic wind, radicals and ozone, which was found to eliminate machining heat very effectively. Although Blomgren attributed the heat reduction observations mainly to the presence of the electrostatic field (silent discharge, without arcing), the electric wind carrying ionized species and in particular oxygen and ozone have since been demonstrated to play a very important role in friction reduction, hence lowering cutting heat as observed by Blomgren. For example, ozonated air has been demonstrated as a viable dry lubricant means for cutting ferrous steels (Han). Iron oxides provide excellent boundary layer lubrication (Schey Page 41), so it would be expected that ozonated air would produce ample iron oxide lubrication. However, metal oxides derived from nickel, aluminum, titanium, lead and zinc do not work well as boundary layer lubricants due to their extreme hardness, much higher than the nascent metal from which they have been formed. In these applications, special cutting tools and coatings and/or lubricants must be employed.
The present inventor has developed machining fluid inventions utilizing carbon dioxide, a mole of which contains two moles of oxygen. One exemplary invention that embodies the relevant prior art in this regard is U.S. Pat. No. 7,389,941. The '941 patent includes a nozzle device and method for forming a composite fluid. The nozzle device generally comprises a nozzle portion connected to a main body. The main body includes an inner axial bore extending there through. The apparatus contains a tube for transporting carbon dioxide particles within the axial bore of the main body and nose section, terminating at an exit port of the nozzle section. A second tube for transporting lubricants and additives disposes within the axial bore of the main body, terminating proximate the portal. A propellant fluid (typically compressed air) introduced under pressure into the bore of the main body directs both the carbon dioxide particles and lubricants toward the cutting zone. An electrostatic charging device is taught for enhancing the vortical mixing and coalescence the carbon dioxide particles and lubricant additives in transit and into the cutting zone. A drawback of this invention is that both carbon dioxide coolant and oxygenating fluid (compressed air) consumption can be very high (and subsequently very expensive); thus applications using the '941 invention are fairly restricted to relatively low volume and extremely hard to machine applications that can demonstrate a significant return-on-investment due to improvement in machining productivity. The present inventor believes that a lack of performance by CO2 based fluids for this particular application is due to one or a combination of the following factors−1) the energy needed to break the carbon-oxygen bond in CO2 is very high (783 kJ/mole) compared to oxygen (498 kJ/mole); 2) CO2 shields much of the oxygen gas needed for proper surface lubrication from the cut zone, 3) slower oxidation rate of nascent stainless steel (and other high Ni or Cr alloy) surfaces, and 4) cooler operating temperatures afforded by CO2 sublimable coolant particles. A lack of oxygen and too low of cutting temperature in the cutting zone is known to negatively affect steel machining performance. Conventional machining fluid processes using air-oil and flooded air-oil-water coolant and operating at much higher cutting zone temperatures have demonstrated as good as or better performance (i.e., tool life) than carbon dioxide fluids of '941 in these particular machining applications. For example, experiments performed using CO2 particles and oil (Nguyen) demonstrated low performance in hard grinding processes due to high cost of CO2 and “oxygen starvation” in the working environment. Nguyen determined that cold air and oil provided lower grinding forces, presumably due to enhanced oxygenation of the grinding/cutting zone. The present inventor has experienced the same negative result in hard grinding applications utilizing the '941 invention.
Another more conventional method of oxygenating a machining fluid is to mix small quantities of lubricant with air, called minimum quantity lubrication (MQL). The conventional method of MQL, for example the application of a spray emulsion of water-oil, involves conveying the fluid to the tip of a suitable nozzle which is subsequently atomized to form an aerosol comprising gas-liquid particles. The resulting MQL aerosol spray is directed upon the surfaces being machined. Upon contact, the result is the evaporation or expansion of the more volatile phase (water-gas), providing cooling to the cutting edge and machining tool and the deposition of the less volatile phase (oil and additives) as a film which provides the lubricity necessary for efficient cutting and longevity of the cutting tool. However, a drawback of this approach is that the air is only 20% oxygen and 80% other gases that do not contribute oxides. Another drawback is the low solubility of oxygen in lubricating fluids such as oils, thereby not readily dissolving in solution.
In studies comparing flood and MQL machining, it has been found that the oxygen content is critical to the performance of the lubricant (Weinert, Inasaki). It was determined one study that an increase in oxygen content in the vicinity of the vegetable oil ester increased lubricant adsorption on the polar metal surfaces, which lowered the friction coefficient. Thus oxygen content in oil, whether delivered as a flood or MQL, is a key process variable. Moreover, similar to oxygen functionalization, organic sulfur groups present on the oil molecule itself have been proven to lower friction (hence heat) in metal cutting operations (Sharma).
Still moreover, the efficient and effective application of conventional MQL sprays to machined articles by mechanical atomization presents several challenges. When sufficiently high spray velocities are employed to provide enough energy to reach cutting zone surfaces, the majority of the spray tends to deflect from or stream around surfaces rather than impinge upon them. When low velocity atomization is employed, critical surfaces with recesses or complex surfaces cannot be penetrated effectively. Also, it is known that oil droplets, evenly finely atomized, tend to agglomerate into larger droplets during transition from spray nozzles to surfaces. This phenomenon interferes with the even distribution of coolants and lubricants on machined surfaces and causes a large portion of the atomized spray to missed the substrate entirely if positioned at location far away from the substrate being machined, wasting a portion of the applied spray. Still moreover, conventional atomized MQL sprays can create a fog or fine mist, which has been determined to be hazardous to workers and is considered an in-door air pollutant.
Methods to improve atomization and minimizing “fog” creation have been developed. For example, Trico Manufacturing Company, Pewaukee, Wis. has developed a microdispensing system. In this system, lubricating oil is introduced into the interior region of flow nozzle using a separate delivery tube and air which flows along the outside of the nozzle gently break-ups the oil droplets and incorporate them into the airstream. This type of system is described fully in an article entitled “Micro-Improvement: Advanced Lubricants and Nozzles Improve Fluid-Mist Systems”, by B. Rake, Cutting Tool Engineering Magazine, April 2002, Volume 54, Number 4.
Other conventional MQL delivery methods and apparatuses are described in U.S. Pat. Nos. 5,002,156, 5,333,640 and 5,444,634, and also as available as the AccuLube (Trademark of ITW) Precision Lubricant Applicator system available commercially from Illinois Tool Works, Rocol Division, Glenview, Ill.
In an article entitled “Dry Goods: Factors to Consider when Dry or Near-dry Machining”, Greg Landgraf, Cutting Tool Engineering, January 2004, Volume 56, No. 1, several major MQL challenges are cited by experts in the field which have heretofore not been adequately addressed. Cited challenges include 1) lubricating the exact cutting edge, 2) inability of mist to make it around sharp edges, 3) preventing coolant-lubricant from collecting in the delivery lines and spray nozzles, 4) fine sprays do not work well with fast moving tools or substrates and 5) inability to evenly coat the cutting edge or workpiece, among several other related challenges. Moreover in an article entitled “In the prior art, dielectric gases and particles have been charged principally by two methods; 1.) Charging using a corona discharge and 2.) Charging by contacting them with one another or surfaces having different dielectric properties, a phenomenon called passive charge transfer or tribocharging. Electrical-energy induced electrostatic processes (corona charging processes) have been employed in a variety of commercial spray applications to beneficially improve transfer efficiency. These are described in detail in U.S. Pat. Nos. 2,302,289, 2,894,691, 5,056,720, 5,312,598, 5,591,412, and 6,105,886. Moreover tribocharging processes are described in U.S. Pat. No. 5,402,940, as well as combinational corona and tribocharging system as described in U.S. Patent Application Publication US 2004/0251327 A1. These systems have been used for example to coat metals with polymeric paint particles, a process called powder coating, wherein the electrostatically charged particles are entrained and delivered to a surface in a secondary fluid such as compressed air.
Electrostatic fields have also been used to charge a lubricating oil to coat metal surfaces. In U.S. Pat. No. 4,073,966 Scholes et al teach corona charging a lubricating oil and delivering this charged lubricant into a machine as a means for improved surface coating for general machine lubrication applications. However this method does not teach using a phase change agent to accomplish the inductive charging or directing the electric field generated by same into a workpiece surface being cut by a machining tool. Moreover, the charged oils in '966 are delivered as charged liquids entrained in a propellant gas, and not means for increasing particle density due to phase change of the charged oil droplet is taught such as in the present invention. Another significant limitation of this approach for metalworking applications is the poor heat capacity of the oil or propellant gas for most material cutting applications discussed herein.
Other electrostatic spraying methods have been developed, called “electrohydrodynamic spraying”. Electrohydrodynamic spraying conveys and coats surfaces with small quantities of dielectric fluids such as oils without the need for a secondary fluid conveyance stream. These are described fully in U.S. Pat. Nos. 3,648,401, 4,341,347, 4,749,125 and 4,776,515. A commercial electrohydrodynamic spraying system described in U.S. Pat. No. 4,749,125 is available from Terronics Development Corporation, Elwood, Ind.
The application of electrostatic fields to cool cutting operations as previously taught by Blomgren are part of a larger discipline called Electrohydrodynamics (EHD), also known as electro-fluid-dynamics (EFD) or electrokinetics. EHD is the study of the dynamics of electrically conducting fluids. It is the study of the motions of ionized particles or molecules and their interactions with electric fields and the surrounding fluid. In general, this phenomenon relates to the direct conversion of electrical energy into kinetic energy, and vice versa. In the first instance, shaped electrostatic fields create hydrostatic pressure (or motion) in dielectric media, such as air. When such media are in a gas or liquid state, such as when a solid carbon dioxide particle changes into a liquid upon impact within a cut zone, electro-capillary flow is produced which enhances penetration, wetting and lubrication, and heat extraction. Moreover, charged aerosols directed against an interface (i.e., conductive workpiece and dielectric coated cutting tool (i.e., diamond), representing electrode pairs in effect, generates current flow such as in an electric motor. Such electrical flows produce energy flow (heat, electrons) at the interface to reduce tool wear.
EHD cooling aspects have been studied elsewhere. For example, in an article entitled “The effect of an electric field on boiling heat transfer of refrigerant-11-boiling on a single tube”, Kawahira, H.; Kubo, Y.; Yokoyama, T.; Ogata, J. Industry Applications, IEEE Transactions, Volume 26, Issue 2, March/April 1990 Page(s):359-365, The effect of an electric field on boiling refrigerant R-11 was investigated experimentally. The test section consisted of a flat plate and a single tube with several rows of electrode wire. In the tests performed, it was found that as the applied voltage increased, the number of boiling bubbles decreased but the heat transfer coefficient increased, and the polarity of the applied voltage affected the boiling heat transfer.
In an article entitled: “Motivation and results of a long-term research on pool boiling heat transfer in low gravity”, P. Di Marco and W. Grassi, LOTHAR (LOw gravity and THermal Advanced Research Laboratory), Dipartimento di Energetica “L. Poggi”, Università di Pisa, Italy, 20 May 2002, It was experimentally shown that the application of an external electric field generally enhanced the heat exchange between a heated wire and a liquid pool. The single-phase heat transfer coefficient was improved, the nucleate boiling region was extended to higher heat fluxes, by increasing the critical heat flux, CHF, as well as the heat transfer rate in nucleation film boiling is augmented.
A beneficial aspect of EHD phenomenon is the suppression of film boiling on hot surfaces. Nucleation film boiling generally occurs with much more difficulty in the interior of a uniform substance such as liquid carbon dioxide, by a process called homogeneous nucleation. Liquids cooled below the maximum heterogeneous nucleation temperature (melting temperature), but which are above the homogeneous nucleation temperature (pure substance freezing temperature) are said to be supercooled. The creation of a nucleus implies the formation of an interface at the boundaries of the new phase. Some energy is expended to form this interface, based on the surface energy of each phase. If a hypothetical nucleus is too small, the energy that would be released by forming its volume is not enough to create its surface, and nucleation does not proceed. As the phase transformation becomes more and more favorable, the formation of a given volume of nucleus frees enough energy to form an increasingly large surface, allowing progressively smaller nuclei to become viable. Eventually, thermal activation will provide enough energy to form stable nuclei. These can then grow until thermodynamic equilibrium is restored. ‘Film boiling’ on very hot surfaces is believed to be stabilized by spontaneous nucleation phenomena, which produces regions having low heat transfer (i.e., vapor pockets) and thermally insulated the pure fluid. This also occurs in machining fluids, where the coolant contacts the hot tool or workpiece surfaces, in particular the cut zone, and instantly forms a thermally-insulative vapor layer which disrupts beneficially cooling effects and prevents lubricants from entering the boundary layer. EHD can be used to disrupt or destroy these boundary layer films thereby dramatically increasing heat transfer and lubrication in machining.
EHD has been exploited commercially to enhance machining. For example, in U.S. Pat. Nos. 3,670,606 and 3,862,391, and more recently in U.S. Pat. No. 7,198,043, all exploit EHD phenomenon using high voltage to enhance natural heat convention and transfer from hot cutting tool surfaces. However both of these techniques require the employment of separate HV electrodes attached to workpiece and cutting tool or indirect contact with an electrostatic nozzle. The fluid (atmospheric gas) heat transfer is improved, but it offers little in heat capacity due to its small mass. Moreover, no additional boundary layer lubrication is provided between the workpiece and tool surface (i.e., cut zone). In U.S. Pat. No. 3,747,284, liquid coolants are electrostatically charged (without producing an arc) with an accompanying electric field produced between the spray nozzle and workpiece. The limitation of this approach is that the EHD-enhanced machining coolants, regardless of improved heat transfer coefficients provided therein, do not have sufficient boundary layer velocities in many cases to enter the cut zone. Moreover, employing bulk liquids for flood cooling is wasteful and creates waste by-products which must be managed. Still moreover, these approaches improve machining heat removal and retard heat build-up only in a macroscopic sense; tool bodies, workholding fixtures and the workpiece itself. The reactive boundary region involving the cutting interface between the tool, chip and workpiece (where the heat is generated) indirectly benefit from the EHD effect.
The prior art is replete with examples of the role of oxygen (and surface oxidation) to reduce (or increase) cutting force and improving (or decreasing) surface finish during cutting. According to Rehbinder, the immersion of wires of certain metals in non-polar paraffin containing a little oleic acid increases the rate of flow of the metal (cadmium) under a given stress and increases the electrical resistivity. His experiments show that the mechanical effect can be obtained with single crystals of cadmium if the surface is contaminated by a thin oxide layer, which is known to increase the critical shear stress, but not if the surface is clean. The “Rehbinder effect” is attributed to the disruption of the hardening surface layer by the active agent (i.e., oxygen) and not to penetration into the metal. In metal cutting applications such as ferrous metals, the iron oxides formed on these metals during cutting are beneficial. However thermal management is still required to maintain optimal cutting temperatures and additional chemistry may be required to lower critical stress of cutting. For example, ozonated air (direct oxygenation of the cut) can enhance C45 ferrous metal cutting operations, due primarily to the lubricating benefit afforded iron oxides thus formed. However typically, newly oxidized metal surfaces are mostly beneficial for promoting the proper surface chemistry for a secondary lubricant and additives to perform their functions properly. In some materials such as stainless steels, nickel and chromium do not form oxides rapidly and if formed can be detrimental to machinability due to the higher shear stresses of chromium and nickel oxides. Moreover the presence of other extreme pressure stress and strain reduction compounds such as sulfur may be needed for a particular cutting operation.
As such, there is a present need for an improved cooling and lubricating method that can be adapted to or dynamically “tuned” in-situ to optimize the cutting conditions of a particular machining application, and for particular cutting tools and coatings and for the improved machining, cutting or grinding of a variety of advanced materials under flood, dry and near-dry conditions. It is, therefore, a primary object of this invention to provide a novel method and apparatus for the functionalizing an oil (bio-based preferred) in-situ with oxygen, and non-toxic organic sulfur too (as an extreme pressure agent), and controllable oxygenation and sulfurization levels of same. A secondary object of the present invention is to provide an improved method for delivering and applying said “oxygenated oils” using a more effective, economical and versatile cooling-lubricating apparatus to meet the demands of a diversity of machining, cutting, grinding operations and which provides a multitude of performance, worker safety and environmental benefits in a wide variety of machining applications.